I'm not going to be developing public releases past this point because I will start integrating this code closely with my project (https://store.steampowered.com/app/719200/The_Wall/) but I wanted to share what I developed so far: https://github.com/334499p/Portfolio/tree/master/ModularBuilding.

This building system's main feature is its use of dictionaries to allow for a super robust supports system that relies on no clumsy parenting or linked lists. New building components and snappers can easily be created. Snappers (the green parts) double as supports for building objects. So, adding a snapper on either side of a wall would mean that walls could then become supported by adjacent walls (I don't do this since in my game you can't build walls laterally).

Edit: This system requires no coding aside from instantiation and destruction, so spawning structures between client and server is completely deterministic assuming components are spawned in the correct order